The present invention relates, in general, to powered apparatus utilizing a drive nut working in cooperation with the drive screw to move a load in a direction substantially parallel to the drive screw and, more particularly, the present invention relates to powered door operators for passenger transit vehicles utilizing drive screw and drive nut assemblies to move a door for covering and uncovering an aperture for ingress and egress of passengers in a wall of a transit vehicle.
Drive screw and drive nut arrangements disposed within a powered apparatus move a load connected to the drive nut through a drive linkage in a predetermined direction, generally, substantially parallel to the axis of such drive screw. Lubricants are generally applied to the drive screw to minimize frictional forces and increase the working life of the drive nut, furthermore, the lubricants are selected and approved to work in a particular environment and work in cooperation with the particular materials of the drive screw and drive nut. Since the lubricant properties are altered during load movement, these lubricants are generally re-applied on a periodic time basis, generally preceded by removal of the old lubricant from the drive screw and drive nut. The re-lubrication effort may be time consuming and requires availability of the proper lubricants at the time of the effort. It is well known that unapproved lubricants have been used when the primary lubricants are not available. Examples of such apparatus generally include an apparatus for moving a spindle of metal cutting equipment, an apparatus for moving a work holding table of such metal cutting equipment, a powered apparatus for moving a door for covering and uncovering an aperture of a building and a powered door operators for passenger transit vehicles utilizing drive screw and drive nut assemblies to move a door for covering and uncovering an aperture for ingress and egress of passengers in a wall of a transit vehicle. The use of it in conjunction with the present invention will be explained as used in a particular application within a mass transit vehicle.
It is generally well known in the passenger transit vehicle art to employ a door drive mechanism which includes a drive screw engaging a drive nut that is attached to a transit vehicle door so that when the drive screw is rotated such drive nut is moved longitudinally along the drive screw in order to open and close the door. The drive nut is usually connected to the door panel or door panel hanger via an elaborate linkage to minimize the substantial force generated by offsetting the door drive and door hanger cavity.
U.S. Pat. No. 6,026,697 teaches a drive nut bracket for attaching a drive nut to a door panel in order to move the door panel in a drive direction. The drive nut is engaged with a drive screw which is aligned substantially parallel to the drive direction. When the drive nut moves as the drive screw is rotated the drive nut bracket provides rotational constraint, thus, preventing the drive nut from rotating about the axis of the drive screw. The drive nut bracket also provides linear constraint of the drive nut along the axis of the drive screw between the drive nut and the driven component. Rotation of the drive screw causes motion of the drive nut parallel to the axis of the drive screw and hence causes movement of the driven component in the drive direction.
In this arrangement the drive nut and drive screw arrangement is displaced from the door panel hanger in both the horizontal and vertical planes, thus increasing the space envelope needed for the entire door operator assembly. The physical displacement between the drive member and door panel hanger results in additional adjustments of the door panel with regards to motion transverse to the panel plane and hanger cavity axis and requires complex linkage to minimize loads being applied between the drive nut and the drive screw in directions perpendicular to such drive screw. This is particularly the case in a passenger transit vehicle which is not built to close dimensional tolerances and which may also undergo flexure due to a load of passengers.
U.S. Pat. No. 6,026,697 furthermore teaches a one-piece drive nut having a threaded bore for engagement with a drive screw to be driven thereby. A threaded bore is manufactured to close tolerances in order to provide proper fit between the drive nut and the drive screw.
It is well known in the art of the door operators for passenger transit vehicles employing a drive screw and drive, that the outer surface of the drive screw is coated with lubricants to reduce friction forces between the drive nut and drive screw due to close tolerances. During operation, the door operator and, more particularly, drive nut and drive screw interface is subjected to contamination which find access into the interior structure of the vehicle through the door panel interface with the car structure and various wire ducts carrying electrical wiring. As this contamination settles on the exterior surface of the drive screw it is attracted by the viscosity of the lubricant structure and impregnates and alters lubricant structure resulting in increased friction between the drive nut and drive screw and in additional loads acting on the drive screw. Furthermore, lubricants must be re-applied on a periodic basis thus increasing maintenance costs. Traditionally, application of the drive nut and drive screw without the use of lubricants result in drive nut wear requiring the replacement of such drive nut which necessitates significant disassembly of the door operator mechanism.
Prior art systems have a significant amount of mechanical backlash, which creates problems for the control of the doors and for the detection of obstructions between door panels.
As it can be seen from the above discussion it will be advantageous to minimize the displacement between the door panel hanger and the drive screw and resulting wear producing forces, improve door control, and eliminate the need for lubricants while minimizing replacement effort of the drive nut.
A powered door operator for passenger transit vehicles, having a drive nut assembly engaging a door hanger bracket substantially connected to a door disposed within an aperture of the vehicle structure. The drive nut further engaging a drive screw to move a door for covering and uncovering an aperture for ingress and egress of passengers in a wall of a transit vehicle upon rotation of such drive screw enabled by a rotary prime mover. The drive nut assembly comprises a housing having an internal cavity, at least one drive nut insert having a helix portion substantially identical to the drive screw helix and disposed within such internal cavity and a retaining clip engaging the at least one drive nut insert for substantial retainment thereof. The retaining clip further provides means for simple replacement of such at least one drive nut insert during maintenance cycles thereby substantially eliminating periodic lubrication of the drive screw. Such drive nut assembly further comprises spherical bushings attached to a force transmitting portions disposed within a drive nut housing and engaging force receiving portions disposed within such door hanger bracket. Means are provided to substantially minimize side loads acting on a drive screw due to drive rod bending and deflection during door motion. Further means are provided to compensate for shock and vibration acting on a drive screw and drive nut during door motion.
It is, therefore, one of the primary objects of the present invention to provide a drive nut arrangement that substantially eliminates the need for lubricants to be applied to the outer surface of the drive screw.
It is a further object of the present invention to provide a drive nut arrangement that substantially minimizes door drive/door offsets and wear producing forces.
It is another object of the present invention to provide a drive nut arrangement that provides for ease of replacing worn components.
It is an additional object of the present invention to provide a drive nut arrangement that minimizes door operator maintenance cost.
It is yet another object of the present invention to provide a drive nut arrangement that substantially minimizes mechanical backlash.
In addition to the various objects and advantages of the present invention which have been generally described above, there will be various other objects and advantages of the invention that will become more readily apparent to those persons skilled in the relevant art from the following more detailed description of the invention, particularly, when the detailed description is taken in conjunction with the attached drawing figures and with the appended claims.